The present invention relates in general to controlling a cooling fan in an audio system, and, more specifically, to running the cooling fan in a manner that masks audible noise generated by the cooling fan.
Amplifiers used in audio systems cannot be designed with 100 percent efficiency and, therefore, always generate heat. Other components of the audio system, including any microcontrollers or other integrated circuits, media player mechanisms (e.g., CD or cassette tape), displays, transistors, or other discrete components, also generate heat during operation. Heat can especially be a problem in automotive audio systems in which all these elements are housed within a single module or box. Heat buildup can lead to improper circuit operation or failure, audio distortion, undesirable surface heating, or heating of the audio media (e.g., a CD ejected from the system may be hot to the touch). Consequently, heat dissipation or removal is an important consideration for any audio system.
Heat exchange with ambient air is the primary method of removing heat from a typical audio system. In order to increase the heat exchange, a cooling fan may be provided for directing a flow of air through the audio system module so that fresh air is drawn from outside the module, over the heat-generating components, and back outside the module.
A drawback of using a cooling fan in an audio system is the noise generated by the fan itself. Unless great care and expense are taken in designing and manufacturing the cooling fan, the level of noise generated can be considered objectionable by many users of the audio system. Even when cost is not a limitation, all noise cannot be eliminated. Furthermore, damage or structural or electrical changes over time may increase the noise level that is generated.
Annoyance from fan noise can be minimized by running the fan intermittently, such as only when a certain amount of heat has built-up in the module. Many audio power amplifiers incorporate temperature sensing in their integrated circuits, but these typically only generate an output signal when the sensed temperature nears a level where a failure or improper operation may result (most audio systems use this signal to foldback or reduce the audio volume so that less heat is generated in the amplifier). Therefore, turning on the cooling fan in response to this signal is too late to prevent undesirable temperature levels. A separate temperature sensor can be used to sense the temperature within the module, but this adds an undesirable expense. Furthermore, intermittent operation of the cooling fan does not address the problem of audibility of the fan noise when the cooling fan is turned on.
The present invention has the advantage of providing a cooling fan control strategy that operates the fan so that audibility of fan noise is minimized. The present invention uses the masking-effect of loud audio passages to reduce the audibility of the fan noise.
In one aspect of the invention, an audio system for reproducing audio signals comprises an amplifier for amplifying the audio signals. A power monitor determines an output power of the amplifier and generates a power signal proportional to the output power. A switchable cooling fan is positioned to provide a cooling flow of air within the audio system when the cooling fan is turned on. A fan control is coupled to the power monitor and the switchable cooling fan. The fan control compares the power signal with a predetermined threshold and turns on the switchable cooling fan in response to the comparison indicating that reproduction of the amplified audio signals is at an output power level that will substantially mask audible noise created by the switchable cooling fan. Once the switchable cooling fan is turned on, the fan control keeps it turned on for a predetermined period. Even after the predetermined period, the switchable cooling fan is not turned off until the comparison indicates that reproduction of the amplified audio signals will no longer substantially mask audible noise created by the fan.